A major goal in the synthesis of polymides has been to prepare aromatic polyimides having the desired physical and thermal properties to permit dependable aerospace utilization. Recent research has been directed towards developing polyimides that are soluble in common organic solvents, readily processable, and thermally curable without the evolution of volatile by-products. See, for example, Mittal, K. L., "Polyimides: Synthesis, Characterization, and Applications", New York: Plenum Press, 1984; Cassidy, P. E. "Thermally Stable Polymers: Synthesis and Properties", New York: Marcel Dekker, Inc., 1980; and Hergenrother, P. and T. L. St. Clair, "Proceedings of Second International Conference on Polyimides", in Ellenville, N.Y., 1985.
The melt processability of these polyimides can be enhanced by incorporation of aryl-ether and meta-phenylene linkages within the polymer backbone.
One method of synthesizing aromatic polyimides utilizes the Diels-Alder polymerization of bis(3,4-dimethylenepyrrolidyl) arylenes and bismaleimides (see Ottenbrite, R. M., Yoshimatse, A., and Smith, J. G. Poly. Prepr., 1987, Vol 28 (2), 280; and Ottenbrite, R. M., and Smith, J. G., Poly. Prepr., 1988, Vol 29 (1) 263). A problem with this monomer system is that it is very reactive; thus, making purification and storage difficult. Also, the polymers prepared from this system were soluble only in concentrated sulfuric acid.
To reduce the reactivity of this bis(exocycliediene), one of the pyrrolidyl ring bonds holding the diene in the reactive s-cis configuration was eliminated by the synthesis of bis(N-isoprenyl) tertiary aromatic amines (see Ottenbrite, R. M. and Smith, J. G., "Preparation of Polyimides from Di(isoprenyl) Diamines", Poly. Prepr., 1989, Vol 30 (1). These monomers were observed to be less reactive thus, allowing for their purification and storage. However, polymers prepared from this monomer system were of low molecular weight.
To increase the reactivity of the bis(N-isoprenyl) monomer, steric bulk was introduced in the ortho positions to the aromatic amine functionality by the synthesis of 1,4-N,N'-diisoprenyl-2,3,5,6-tetramethyl benzene (see Ottenbrite, R. M. and Smith, J. G. "Polyimides Prepared from Bis(isoprenyl) of Sterically Hindered Aryl Diamines", Poly. Prepr., 1989, vol 30 (1).) Results from the inherent viscosity data indicates that this monomer series was more reactive than those previously reported, however the resulting polymer is easily oxidized as evident by the TGA data in air. In the present invention, it is hypothecated that this oxidation may be occurring at the secondary amine site, and the amidation of the bis(N-isoprenyl) secondary amine monomers is effected to eliminate the easily oxidized amine group and to further increase the reactivity of the bis(N-isoprenyl) monomer.